Zirconium ferrite incorporated zeolitic imidazolate framework-8: a suitable photocatalyst for degradation of dopamine and sulfamethoxazole in aqueous solution

The complete removal of pharmaceutical wastes from polluted water systems is a global challenge. Therefore, this study incorporates zirconium ferrite (ZrFe2O4) into zeolitic imidazolate framework-8 (ZIF-8) to form ZrFe2O4@ZIF-8. The ZrFe2O4@ZIF-8 is a photocatalyst for removing dopamine (DOP) and sulfamethoxazole (SMX) from an aqueous solution. The scanning electron micrograph revealed the surfaces of ZrFe2O4 and ZrFe2O4@ZIF-8 to be heterogeneous with irregularly shaped and sized particles. The transmission electron micrograph (TEM) images of ZrFe2O4 and ZrFe2O4@ZIF-8 showed an average particle size of 24.32 nm and 32.41 nm, respectively, with a bandgap of 2.10 eV (ZrFe2O4@ZIF-8) and 2.05 eV (ZrFe2O4). ZrFe2O4@ZIF-8 exhibited a better degradation capacity towards DOP and SMX than ZrFe2O4. ZrFe2O4@ZIF-8 expressed a complete (100%) degradation of DOP and SMX during the photodegradation process. Interestingly, the process involved both adsorption and photocatalytic degradation simultaneously. ZrFe2O4@ZIF-8 demonstrated high stability with a consistent regeneration capacity of 98.40% for DOP and 94.00% for SMX at the 10th cycle of treatment in a process described by pseudo-first-order kinetics. The study revealed ZrFe2O4@ZIF-8 as a promising photocatalyst for the purification of DOP and SMX-contaminated water systems.


Introduction
The inability to completely remove pharmaceutical wastes from drinking water sources is a global problem. Different classes of pharmaceutical wastes have been detected in surface and underground water systems, wastewater treatment discharges and domestic wastewater. [1][2][3][4] The pharmaceutical wastes in water are toxic emerging contaminants because of their capacity to become a threat to human beings and aquatic life. Their presence in water is undesired, and it is crucial to eliminate them. Therefore, this study focuses on developing a means to remove toxic pharmaceutical contaminants in water. Dopamine (DOP) and sulfamethoxazole (SMX) are examples of undesired pharmaceutical contaminants detected in water. 5,6 The continuous use of DOP in chemical synthesis has contributed immensely to its presence in the laboratory and industrial effluents. DOP and SMX are readily available and are easily purchased without a prescription in most developing countries.
The continuous use of SMX in treating ailments in human and animal husbandry has aided its frequent occurrence in drinking water sources. SMX is a known antibiotic for treating infections. 7-9 SMX is very stable (thermal and photostability) in the environment, which gives it a prolonged presence when it gets into drinking water sources, making it possible for it to be transported from one point to another. The persistence of SMX in the environment has contributed to the emergence of drugresistant strains of pathogenic organisms. 9,10 The emergence of drug-resistant pathogens is a serious global challenge with many concerns. 11 One of the ways to address these concerns is to develop a means for the complete removal of active drug species in water systems. Previous studies 12,13 reported SMX in surface water (0.94 mg L −1 ), effluent discharges (24.81 mg L −1 ) and potable water (12.00 mg L −1 ). Most studies have reported the concentration of SMX in the environmental water system to vary from ng L −1 to mg L −1 . [14][15][16] . Even though SMX remains one of the early detected antibiotics in water, its complete removal in water is still a challenge.
Besides being a neuromodulator for treating conditions such as Parkinson's disease, DOP is used for several syntheses. It has been used to prepare nanocomposites [17][18][19] and other improved products. 20,21 When used during synthesis, they are generated into laboratory waste and discarded in laboratory effluent. Many biochemical laboratories in tertiary institutions and research institutes use DOP during practical sessions in which they get into wastewater generated. Most of these laboratories need more capacity to remove DOP in wastewater generated entirely. Furthermore, the presence of DOP biomarkers has been reported in a wastewater-based epidemiology study. 22 Other studies suggest its presence in the environment. [23][24][25] When present in an environmental water system and under certain environmental factors, both DOP and SMX could metamorphose into new compounds which may be hazardous to humans and the environment suggesting their immediate removal.
Many methods have been reported to remove SMX in water systems. 26,27 Recently, a study reported the synthesis of a ternary LTO/CN/AgI nanohybrid catalyst with multicharged transfer channels for the degradation of SMX. 28 The catalyst demonstrated a high kinetic rate constant of 0.25776 min −1 for the degradation process with an insight at the molecular level. Some authors combined hydrothermal and photodeposition as methods for the preparing Ag/g-C 3 N 4 (CN)/Bi 3 TaO 7 (BTO) as photocatalyst for the degradation of SMX under the inuence of visible-light. Although there was an improved performance but there was no complete removal (100%) of the SMX in solution. 29 Furthermore, Co doped ZnO nanorods and other some other potential photocatalysts have shown capacity as efficient catalyst for water purication [30][31][32] while AgNbO 3 corroborated this fact under visible light with an impressive performance with without still attaining complete removal of SMX in solution. 33 Unfortunately, these methods have shown some drawbacks that could be more improvable. The major drawback of these methods is the inability to remove SMX from contaminated water systems completely. On the other hand, there are limited studies on removing DOP in water. It is crucial to investigate the removal of DOP from aqueous solution due to its frequent use and entrance into the environment. Photocatalysis remains an effective method for removing organic molecules in polluted water systems. [34][35][36] The photocatalysis process involves using a photocatalyst to promote the oxidation of organic molecules in water to CO 2 and H 2 O. Some studies have reported using nanoparticles, such as semiconductors, to remove organic molecules in water. [37][38][39] Moreover, such semiconductors can also be photocatalysts for photodegrading organic contaminants in water. Sadly, some semiconductors are expensive or limited in their activity in the visible light region. It is essential to use photocatalysts with efficient action in the visible light region to reduce process costs since visible light is freely available. Therefore, this study suggests zirconium ferrite (ZrFe 2 O 4 ) as an effective photocatalyst in the visible light region.
ZrFe 2 O 4 is of interest because of its unique properties, such as small size, thermal stability, optical properties, and electrical properties. Unfortunately, particles of ZrFe 2 O 4 aggregate, which causes recombination limiting its photocatalytic activity. Therefore, this study proposes the inclusion of ZrFe 2 O 4 in a metal-organic framework, zeolitic imidazolate framework-8 (ZIF-8), forming ZrFe 2 O 4 @ZIF-8 to circumvent the challenge. In the structure of ZrFe 2 O 4 @ZIF-8, ZIF-8 serves as a carbon source inhibiting the aggregation of ZrFe 2 O 4 particles and enhancing its recovery from solution. Currently, there are limited report on the photodegradation of DOP and SMX by ZrFe 2 O 4 @ZIF-8. The current study, therefore, aimed at achieving the complete removal of DOP and SMX in contaminated water systems using ZrFe 2 O 4 @ZIF-8.

Characterization of ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 particles
The functional groups in ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 were evaluated by taking spectra readings at 400-4500 cm −1 on Fourier-transformed infrared spectroscopy (FTIR, PerkinElmer, RXI 83303, USA). Their thermal stability was analyzed via thermogravimetric analysis (TGA) on TGA/DSC 2 Star e system (DB V1300A-ICTA-Star e ), and the diffraction pattern was recorded using X-ray diffractometer (2q) read at 5-90°with ltered Cu Kb radiation. The activity of ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 in the UVvisible light region was recorded using a UV-visible spectrophotometer, while the surface morphology and elemental composition were determined using SEM (JEOL JSM-5510LV) equipped with energy-dispersive X-ray spectroscopy (EDS) (INCA mics EDX system). TEM images were taken on Talos F200X G2.

Photocatalytic degradation of DOP and SMX by ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF
The removal of DOP and SMX from the solution was achieved via photocatalytic degradation using ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 under visible light with the help of a solar simulator (Xe, 150 W) possessing lter holder. 36 Test solutions (50 mL) of DOP (5.00 mg L −1 ) and SMX (5.00 mg L −1 ) were contacted separately with ZrFe 2 O 4 (0.1 g) or ZrFe 2 O 4 @ZIF-8 (0.1 g) in a beaker (100 mL) under visible light irradiation while stirring at 120 rpm for 180 min. The distance between the test solution and the solar simulator lamp was maintained at 20 cm. Samples of DOP or SMX from the degrading test solutions were withdrawn at an interval to evaluate the degradation capacity of ZrFe 2 O 4 or ZrFe 2 O 4 @ZIF-8. The drawn samples were analyzed using a UVvisible spectrophotometer (PerkinElmer, Lambda). The photodegradation was established aer taking UV-visible measurements at a predetermined wavelength: DOP (l max = 280 nm) and SMX (l max = 257 nm). Based on the better performance of ZrFe 2 O 4 @ZIF-8, further studies on process parameters, including the effect of weight (0.1 to 0.5 g), the concentration of DOP and SMX (1.00 to 5.00 mg L −1 ) and pH (2-10) on the photodegradation of DOP and SMX were only carried out using ZrFe 2 O 4 @ZIF-8. A dark experiment was conducted to check the impact of adsorption on the photodegradation process. The dark experiment included a concentration (DOP or SMX) of 5.00 mg L −1 , a weight of 0.1 g (ZrFe 2 O 4 @ZIF-8) and a solution pH of 7.2 without light irradiation. All the experiments were conducted thrice, and values are presented as a mean of triplicate readings. The degradation efficiency was calculated as follows: where C 0 is the initial concentration of the test solutions of DOP or SMX and C t is the concentration of the test solutions of DOP or SMX at time t. For the dark experiment, the adsorption capacity (q e ) and the percentage removal (% removal) expressed towards DOP and SMX were calculated as follows: where C 0 (mg L −1 ) represents the initial concentration of the test solution of DOP or SMX, C e (mg L −1 ) is the test solution concentrations of DOP or SMX at equilibrium; V (in litre) represents the solution volume, the weight (g) of ZrFe 2 O 4 @ZIF-8 is dened as m, and the adsorption capacity is given as q e (mg g −1 ).

Scavenging of reactive oxygen species
To understand the mechanism of action of ZrFe 2 O 4 @ZIF-8, the role of reactive oxygen species (ROS) during the degradation of DOP and SMX under visible light irradiation was investigated. The investigation estimated ammonium oxalate (AO) as a hole (h + ) scavenger, isopropyl alcohol (IPA) as a scavenger of hydroxyl radical (OH$) and chloroform (CH) representing scavenger of superoxide ion radical (cO 2 − ). The role played by the scavengers during the degradation was determined by separately including each in the test solution at a concentration of 1 mM while conducting the photodegradation process. All the process conditions, such as test solution concentrations of DOP and SMX (5.00 mg L −1 ), a weight of ZrFe 2 O 4 @ZIF-8 (0.1 g), process time (180 min) and solution pH (7.2) for the photodegradation process were maintained.
The average crystallite sizes of ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 are denoted as D, K is a constant (0.89) while l (1.5406 Å) is the X-ray wavelength. The entire width of the diffraction line and Bragg's angle taken at the peak are represented as b and q, respectively. 43 The crystallite size of ZrFe 2 O 4 @ZIF-8 (26.10 nm) is larger than that of ZrFe 2 O 4 (21.23 nm), which may be due to a larger molecular size of ZrFe 2 O 4 @ZIF-8 from the incorporation of ZIF-8 in its structure. This may have caused an extension in the bulk crystallite size, which is described by the diffusion properties exhibited by ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8: 44,45  The average diffusion time to the surface of ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 is s while D is the diffusion coefficient. From the expression, s gets longer when D becomes large; this possibility puts the particles at risk of aggregation or recombination when functioning as a catalyst. The capacity of the particles to act as a catalyst becomes hampered when aggregation or recombination occurs among the particles. 45,46 Therefore, for optimum catalytic performance, the crystallite size should be small. 47 Interestingly, the crystallite size exhibited by ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 is smaller than the range (37 to 45 nm) reported for spinel ferrites, 47 suggesting ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 as potential photocatalysts.
The TGA results showed distinct phase losses in ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 (Fig. 1c). The mass loss at 60 to160°C suggests loss of adsorbed water molecules (peaks at 3243 and 1635 cm −1 in the FTIR results) and volatile molecule adsorbed on the surfaces of ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8. There is a mass loss from 160 to 630°C in ZrFe 2 O 4 , which may be due to the formation of metal oxides and dehydration of the OH group in its spinel structure involving inter and intramolecular transfer reactions. 48,49 The mass loss from 630 to 710°C (ZrFe 2 O 4 ) and 510 to 900°C (ZrFe 2 O 4 @ZIF-8) may be attributed to phase change and decomposition of ZIF-8 structure, respectively. Mass loss above 900°C in ZrFe 2 O 4 @ZIF-8 may be attributed to structural collapse and carbonization, 50 while mass loss above 710°C in ZrFe 2 O 4 may be due to phase change. ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 exhibited activity in the visible light region of the spectrum, as shown in Fig. 1d, indicating that they may both exhibit photocatalytic activity within this region of the light spectrum. This indication led us to probe the possibility of using ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 for the photodegradation of DOP and SMX. The band gaps were calculated from the Tauc plot for ZrFe 2 O 4 (Fig. 2a) and ZrFe 2 O 4 @ZIF-8 (Fig. 2b) as follows: Where hv represents the frequency of light from the solar irradiator, the proportionality constant is dened as A, the bandgap is denoted as E g and a represent the absorption coefficient. The average particle size of ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 were found to be 24.32 nm and 32.41 nm, respectively, from the TEM images. The average particle size increased in ZrFe 2 O 4 @ZIF-8, which may be due to the imidazole structure of the ZIF-8 that increases the molecular weight. The particles exhibit irregular sizes and shapes.

Photodegradation of DOP and SMX
The preliminary performance of ZrFe 2 O 4 and ZrFe 2 O 4 @ZIF-8 (Fig. 4a)   towards DOP was 93.85 ± 0.50% and 90.60 ± 1.00% towards SMX. On the other hand, ZrFe 2 O 4 @ZIF-8 expressed a complete (100%) degradation of DOP and SMX in the test solutions. Therefore, further studies for the degradation of DOP and SMX were conducted using ZrFe 2 O 4 @ZIF-8. The time-dependent degradation of DOP and SMX by ZrFe 2 O 4 @ZIF-8 are presented in Fig. 4b and c, respectively. In both DOP and SMX, the degradation efficiency expressed by ZrFe 2 O 4 @ZIF-8 increased with time. The initial degradation efficiency expressed by ZrFe 2 O 4 @ZIF-8 towards DOP and SMX is higher at low concentration (1.00 mg L −1 ) than at high concentration (5.00 mg L −1 ). The observation may be because at low concentrations, smaller amounts of DOP and SMX species are available in solution for ZrFe 2 O 4 @ZIF-8 to degrade, and as concentration increased from 1.00 to 5.00 mg L −1 , the quantities of DOP and SMX species in solution increased requiring more activities of ZrFe 2 O 4 @ZIF-8 to ensure degradation.
The effect of ZrFe 2 O 4 @ZIF-8 weight on the degradation of DOP and SMX is shown in Fig. 4d. The degradation efficiency expressed by ZrFe 2 O 4 @ZIF-8 towards DOP and SMX increased with an increase in weight of ZrFe 2 O 4 @ZIF-8. This may be due to a rise in the surface area of ZrFe 2 O 4 @ZIF-8 as its weight increased from 0.01 to 0.2 g. Increasing the weight must have increased the number of active sites available for the degradation process, thereby increasing the efficiency of ZrFe 2 O 4 @ZIF-8 as weight increased. Similar observations as been previously reported. 36 Any attempt to increase the weight of ZrFe 2 O 4 @ZIF-8 beyond 0.2 g led to a decrease in the activity of ZrFe 2 O 4 @ZIF-8, which may be attributed to the fact that as the weight of ZrFe 2 O 4 @ZIF-8 increased beyond 0.2 g, the penetration of irradiated light rays reduced. The reduction in light penetration due to the bulkiness of ZrFe 2 O 4 @ZIF-8 as weight increased may have prevented the excitation of ZrFe 2 O 4 @ZIF-8 because of the shielding effect resulting from the excessive scattering of the photons at the surface of ZrFe 2 O 4 @ZIF-8. 53 The role of pH in photodegradation should be investigated because acidity and alkalinity play an essential role in catalyst behaviour in a reaction medium. The test solution pH was varied from 2 to 12 (Fig. 5a) to understand the role of pH in the degradation of DOP and SMX by ZrFe 2 O 4 @ZIF-8. As the pH of the test solution was increased from 2 to 7, the performance of ZrFe 2 O 4 @ZIF-8 was enhanced, and more DOP and SMX were removed from the solution. Unfortunately, the performance of ZrFe 2 O 4 @ZIF-8 decreased as the pH increased aer pH 7.2. Therefore, the best pH for the degradation of DOP and SMX by ZrFe 2 O 4 @ZIF-8 is 7.2. As pH increased towards 7.2, more ROS were available in the test solution for the degradation process. Degradation data were tted for the pseudo-rst-order kinetic model to understand the rate of the degradation process as: where C i and C t are the initial concentrations of DOP and SMX and concentrations of DOP and SMX at a specic time "t", respectively, K denotes the pseudo-rst-order rate constant generated from the plot of ln C i /C t versus time, and t is the irradiation time. The photodegradation rate for ZrFe 2 O 4 @ZIF-8 towards DOP and SMX was determined from the plot of ln C i /C t versus visible light irradiation time at the different concentrations of DOP and SMX (Fig. 5b and c). The photodegradation rate constant expressed for the degradation of DOP increased with a decrease in test solution concentration (5.00 mg L −1 = 0.0243 min −1 , 4.00 mg L −1 = 0.0306 min −1 , 3.00 mg L −1 = 0.0352 min −1 , 2.00 mg L −1 = 0.0375 min −1 and 1.00 mg L −1 = 0.0497 min −1 ). It was also observed in the initial degradation efficiency expressed by ZrFe 2 O 4 @ZIF-8 towards DOP (Fig. 4b). The degradation efficiency was highest for the low concentrations at initial treatment time; furthermore, it took a shorter time for ZrFe 2 O 4 @ZIF-8 to completely degrade DOP at the least concentration (1.00 mg L −1 ) than for the higher concentrations. A similar result was obtained for the degradation of SMX (5.00 mg L −1 = 0.0592 min −1 , 4.00 mg L −1 = 0.0611 min −1 , 3.00 mg L −1 = 0.0619 min −1 , 2.00 mg L −1 = 0.0644 min −1 and 1.00 mg L −1 = 0.0645 min −1 ). It may be concluded that the rate of photocatalytic degradation of DOP and SMX by ZrFe 2 O 4 @ZIF-8 is fastest at low concentrations of DOP and SMX, which may be due to the low amounts of DOP and SMX species in solution at such low concentrations. A dark experiment was conducted to investigate the effect of adsorption on the photodegradation process. Interestingly, ZrFe 2 O 4 @ZIF-8 demonstrated more affinity for DOP than SMX (Fig. 5d). During the dark experiment, the adsorption of DOP by ZrFe 2 O 4 @ZIF-8 increased with an increase in concentration (1.00 to 5.00 mg L −1 ) from 5.60 ± 0.50 to 10.20 ± 0.80%, similarly, in the case of SMX, the adsorption of SMX increased from 3.70 ± 0.50 to 7.40 ± 0.80% with an increase in concentration (1.00 to 5.00 mg L −1 ). The adsorption capacity expressed by ZrFe 2 O 4 @ZIF-8 towards DOP and SMX is 0.51 and 0.37 mg g −1 , respectively. This revealed that adsorption and photocatalysis took place simultaneously while removing DOP and SMX from the solution. In both degradations of DOP and SMX, the percentage removal of DOP and SMX via the adsorption process is less than 15% of the total performance of ZrFe 2 O 4 @ZIF-8.

Proposed mechanism for the photodegradation of DOP and SMX
The role of ROS may explain the degradation of DOP and SMX by ZrFe 2 O 4 @ZIF-8. Previous studies have attributed the photocatalytic degradation of organic molecules to the involvement of ROS. 53 Therefore, to understand the mechanism of action of ZrFe 2 O 4 @ZIF-8 for the degradation of DOP and SMX, the degradation process was separately carried out in the presence of IPA (as a OH$ scavenger), AO (as a h + scavenger) and CH (as a cO 2 − scavenger) as described. 36,54 The performance of ZrFe 2 -O 4 @ZIF-8 in the presence and absence of AO, IPA and CH were compared, as shown in Fig. 6a. The performance of ZrFe 2 O 4 @ZIF-8 was least in the presence of IPA and highest in the presence of CH. This observation was found in the degradation of DOP and SMX, which suggests that OH$ played a signicant role in the degradation of DOP and SMX. The scavenging of the OH$ by IPA led to a substantial decrease in the performance of ZrFe 2 O 4 @ZIF-8 as a photocatalyst for the degradation of DOP and SMX. On the contrary, degradation efficiency was highest for CH among the ROS scavengers studied, which suggests that cO 2 − played the least role in the In the present study, scavenging the OH$ by IPA gave the least performance suggesting that it played a signicant role in degrading DOP and SMX. The mechanism for the degradation of DOP and SMX by ZrFe 2 O 4 @ZIF-8 is via cOH, h + , and cO 2 − generation (Fig. 6b) in the test solution when visible light is shone on the degrading system. During the process, ZrFe 2 O 4 @-ZIF-8 absorbs visible light to generate h + from the valence band (VB) and e − from the conduction band (CB). H + and cOH are produced in the test solution from the reaction of h + with water molecules, and subsequently, cO 2 − is produced from O 2 as a result of the reaction of e − . The generated ROS initiates and propagates the degradation process. Unfortunately, h + and e − oen recombine, leading to loss of ROS generation, which is disadvantageous to the degradation process. The recombination of h + and e − was inhibited with the presence of ZIF-8 in the structure of ZrFe 2 O 4 @ZIF-8. The ZIF-8 serves as a carbon source to slow down the recombination process. During this process, the carbon source (ZIF-8) is an acceptor for trapping the generated h + and e − to inhibit their migration for combination. Therefore, when they are trapped, they become xed at a point, making them less mobile and preventing interaction between the h + and e − , as previously demonstrated in a study where a carbon dot was used as a source of carbon. 53,[55][56][57] This approach helped prevent the premature recombination of h + and e − .

Regeneration for reuse and stability of ZrFe 2 O 4 @ZIF-8
The regeneration of ZrFe 2 O 4 @ZIF-8 for reuse is essential as it helps determine its economic viability and affordability.

Conclusion
The presence of pharmaceutical wastes such as DOP and SMX in water is an emerging global water challenge requiring urgent

Conflicts of interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to inuence the work reported in this paper.